The photo above shows Moqui Marbles in their native habitat of southern Utah. These curious rocks are actually concretions having iron (hematite) rinds.
Very similar rocks, called blueberries, have been observed repeatedly on Mars by the rovers. Click here to see an image taken by the Opportunity rover of the blueberries. Some scientific papers implicate the possibility of life on Mars playing a role in their formation while others do not.
Discussions about the pros and cons of their formation have been quite lively at times. However, the consensus seems to be that both the marbles and the blueberries were created beneath the surface as naturally occurring substances, most likely minerals, precipitated from flowing groundwater. Pictured with the marbles is a Devil’s-Claw cactus (Sclerocactus parviflorus). — Bret Webster